A spectroscopic device is designed to measure a spectrum of target light to be measured, and generally includes: a spectroscopic section for spectrally dispersing target light on a wavelength-by-wavelength (wavenumber-by-wavelength) basis; a light-receiving section for receiving light components spectrally dispersed by the spectroscopic section, i.e., wavelength-separated light components, and outputting signals corresponding to respective intensities of the received wavelength-separated light components; and a calculation section for calculating respective intensities of the wavelength-separated light components (a distribution of intensity with respect to wavelength; a spectrum), based on the signals output from the light-receiving section. As one example of this type of spectroscopic device, there is a spectroscopic device in which a filter whose transmission wavelength varies depending on an incident position along a given one direction is used in the spectroscopic section in order to spectrally disperse target light.
In this spectroscopic device, the light-receiving section, which includes a plurality of light-receiving elements to which respective different wavelength bands are assigned, is disposed in spaced-apart relation to the filter by a given distance. In this case, the light-receiving section reflects a part of incident light because it is incapable of fully photoelectrically converting the incident light, and thereby multiple-reflection occurs between the filter and the light-receiving section. As a result, generally, each of the light-receiving elements in the light-receiving section receives not only a light component in a specific wavelength band assigned to the light-receiving element, i.e., a light component to be essentially received by the light-receiving element, but also a light component in a different wavelength band to be essentially received by one of the remaining light-receiving elements. As measures against this problem, in a conventional device, the filter and the light-receiving section are disposed such that one of them is inclined with respect to the remaining one of them.
For example, the following Patent Literature 1 discloses a spectrograph having an inclined detector window, which comprises: a light source capable of emitting a light beam; an entrance slit capable of transmitting a part of the light beam emitted by the light source to generate a transmitted light beam; a grating capable of diffracting the light beam transmitted through the entrance slit to generate a diffracted light beam to thereby produce a spectrum in an image plane (X′, Y′); a detector for detecting the light beam diffracted by the grating, wherein the detector comprises a window for allowing the light beam diffracted by the grating to be transmitted therethrough, wherein a part of the diffracted light beam generates reflection on the window or between the window and a sensing surface of the detector included in a detection plane (X″, Y″); and at least one inclining means for enabling avoiding interference spectra. The spectrograph is characterized in that the at least one inclining means for enabling avoiding interference spectra comprises the detector window which is inclined. That is, the detector window is disposed in front of and inclined with respect to the sensing surface of the detector to thereby eliminate multiple-reflection which would otherwise occur between the detection window and the sensing surface of the detector.
Meanwhile, for example, for design or production reasons, the filter whose transmission wavelength varies depending on an incident position along a given one direction cannot be realized using a single optical filter element, so that it is generally constructed by combining a plurality of optical filter elements together. This leads to occurrence of multiple-reflection within the filter. That is, multiple-reflection occurs between the plurality of optical filter elements. Thus, in such a spectroscopic device using the filter, even when the filter and the light-receiving section are disposed such that one of them is inclined with respect to the remaining one of them, due to the multiple-reflection within the filter, each of the light-receiving elements in the light-receiving section receives not only a light component in a specific wavelength band to be essentially received by the light-receiving element, but also a light component in a wavelength band to be essentially received by one of the remaining light-receiving elements.